The 125 can figure came from the methodology used in some of the low quality rat studies frequently cited to demonstrate the dangers of aspartame back in the day. I'll see if I can find the specific studies.

This page by the National Cancer Institute provides a pretty decent overview of research on a variety of artificial sweeteners. https://www.cancer.gov/about-cancer/causes-prevention/risk/diet/artificial-sweeteners-fact-sheet

One very recent study (Debras et al., 2022) suggested a link between aspartame consumption and cancer, which I suspect is behind the recent hysteria. Pretty much every other high quality study over the past 50 years found no correlation. If aspartame is indeed a dangerous carcinogen, that fact should be clear through epidemiological data alone, like the 2013 study by Marinovich et al. cited in the article. I lend that study and those like it much greater credibility than one-off cohort studies like Debras.